First-in-human phase 1/1b study to evaluate sitravatinib in patients with advanced solid tumors

Sitravatinib (MGCD516), a spectrum-selective receptor tyrosine kinase inhibitor targeting TAM (TYRO3, AXL, MERTK) and split kinase family receptors, has demonstrated preclinical anti-tumor activity and modulation of tumor microenvironment. This first-in-human phase 1/1b study included sitravatinib dose exploration and anti-tumor activity evaluation in selected patients with advanced solid tumors. Primary objectives included assessment of safety, pharmacokinetics and clinical activity of sitravatinib. Secondary objectives included identifying doses for further investigation and exploring molecular markers for patient selection. In phase 1, 32 patients received 10–200 mg, while phase 1b dose expansion comprised 161 patients (150 mg n = 99, 120 mg n = 62). Maximum tolerated dose was determined as 150 mg daily. Dose-limiting toxicity was reported in 4/28 evaluable phase 1 patients (three at 200 mg, one at 80 mg). In phase 1b, 120 mg was defined as the recommended dose due to tolerability. Treatment-related adverse events (TRAEs) were experienced by 174/193 patients (90.2%); grade ≥ 3 TRAEs in 103 patients (53.4%). Most common TRAEs were diarrhea, fatigue, hypertension and nausea; TRAEs led to treatment discontinuation in 26 patients (13.5%) and death in one patient. Sitravatinib was steadily absorbed and declined from plasma with a terminal elimination half-life of 42.1–51.5 h following oral administration. Overall objective response rate was 11.8% in phase 1b, 13.2% in patients with non-small cell lung cancer (NSCLC) and 4.2% in patients with NSCLC with prior checkpoint inhibitor experience. Sitravatinib demonstrated manageable safety and modest clinical activity in solid tumors. NCT02219711 (first posted August 14, 2014). Supplementary Information The online version contains supplementary material available at 10.1007/s10637-022-01274-y.


Introduction
Receptor tyrosine kinases (RTKs) regulate numerous cellular processes including cell proliferation, apoptosis and migration. Aberrant RTK activation is very common in cancer and represents an important therapeutic target for cancer treatment [1]. The utility of RTK-targeted therapies is well-documented in cancers with appropriate genetic alterations, with many targeted therapies now approved worldwide [1].
Sitravatinib (MGCD516) is an orally available small molecule inhibitor targeting closely related spectrum of RTKs, including TAM family receptors (TYRO3, AXL, MERTK) and split kinase family receptors (vascular endothelial growth factor receptor 2 [VEGF-R2], MET, RET and KIT). Several sitravatinib targets, such as TAM receptors, MET, RET and KIT, are dysregulated in many types of cancer through overexpression or genetic alteration, and contribute to tumor development [2]. Additionally, it is well-known that VEGF and its receptors can drive tumor angiogenesis in cancer [3]. Therefore, by targeting this collection of RTKs, sitravatinib may have meaningful anti-tumor effects.
The potent inhibitory activity of sitravatinib was demonstrated with biochemical half-maximal inhibitory concentration values ranging from 1.5-20 nM against target RTKs, including AXL, MERTK, VEGF-R, KIT and MET [4]. Additionally, sitravatinib has demonstrated anti-proliferative effects against solid tumor cells with a variety of phenotypes in vitro, as well as potent anti-tumor activity in xenograft tumor models of lung cancer and sarcoma with RTK dysfunction [4,5].
Here, we report results for the first-in-human phase 1/1b study of sitravatinib, in patients with advanced solid tumors, including non-small cell lung cancer (NSCLC; clinicaltrials.gov identifier: NCT02219711) [6].

Study design
This study was a multicenter, phase 1/1b clinical trial evaluating the safety, pharmacokinetics (PK) and clinical activity of sitravatinib (free base formulation) in patients with advanced solid tumors. The study comprised two main parts: (1) dose escalation (phase 1); and (2) evaluation of clinical activity in patients selected based on histological diagnoses and/or the presence of defined molecular markers (phase 1b).
In the PK lead-in period, patients received a single oral dose of sitravatinib (10-200 mg) under fasted conditions with at least 200 mL of water, followed by PK sample collection for 3-7 days, depending on emerging PK information. After the PK lead-in period, patients commenced the daily regimen planned for their cohort. Blood samples were collected pre-dose and 0.5 (for 10 mg only), 1, 2, 4,  6, 8, 12, 24, 36, 48, 72 and 168 (for 20-200 mg dose levels) h post-dose from patients following a single oral dose, and at pre-dose, 0.5, 1, 2, 4, 6, 8, 12 and 24 h post-dose following multiple oral doses for the 10-200 mg levels.
The starting dose for the phase 1 dose escalation study was 10 mg once daily (QD). Dose escalation was carried out using the modified toxicity probability interval (mTPI) method [7] with the maximum tolerated dose (MTD) defined as the dose associated with a risk of dose-limiting toxicity (DLT) in 30 ± 5% of patients during the first treatment cycle.
Phase 1b cohorts were organized by diagnosis (renal cell carcinoma [RCC]

Choice of starting dose
The starting dose of 10 mg QD was selected based on nonclinical, 4-week toxicology studies conducted in rats and dogs. In rat studies, 10 mg/kg was the highest dose that did not exceed the severely toxic dose in 10% of the animals (STD 10 ). The proposed human dose was based on one-tenth of the STD 10 in rats corrected for body surface area (mg/m 2 ).

Patient eligibility
Eligible patients were ≥ 18 years old with a histologically confirmed advanced, unresectable or metastatic solid tumor for which standard treatment was not available. Eligible patients had discontinued their most recent prior therapy ≥ 2 weeks before their first dose of study treatment and had recovered from any adverse events (AEs) of their prior therapy back to baseline or grade 1 (excluding alopecia); they also had an Eastern Cooperative Oncology Group performance score (ECOG PS) of 0-2.
Patients included in phase 1b cohorts had a selected diagnosis or tested positive for a designated target tumor molecular marker. The following populations were included: patients with NSCLC with a qualifying molecular alteration in MET, AXL, RET, NTRK, DDR2, KDR, PDGFRA, KIT or CBL; patients with other solid tumor types with a qualifying molecular alteration; patients with clear cell RCC (ccRCC) refractory to angiogenesis inhibitors; and patients with metastatic CRPC (mCRPC) with bone metastases.
Patients with symptomatic or uncontrolled brain metastases and/or with a second active cancer (excluding basalcell carcinoma or cervical intraepithelial neoplasia) were excluded. For the phase 1b part, patients who had received prior treatment targeting the molecular marker of interest or patients with ccRCC or mCRPC previously treated with cabozantinib were excluded. Further eligibility and discontinuation criteria can be found in the Supplementary Information (Sects. 1.1 and 1.2, respectively).

Study objectives and assessments
The primary objectives were to characterize the safety profile, PK and clinical activity of sitravatinib. The secondary objectives included exploration of potential pharmacodynamic (PD) markers in blood plasma, identification of doses and regimens of sitravatinib for investigation of clinical activity and exploration of the use of molecular markers for the selection of patients with increased potential for response to sitravatinib.
Safety assessments included evaluation of DLTs, AEs, physical examinations, vital sign measurements, electrocardiogram recordings and laboratory tests. AEs, including laboratory abnormalities, were graded using the National Cancer Institute Common Terminology Criteria for Adverse Events version 4.03 from the day of the first dose of study treatment until ≥ 28 days after the last dose.
PK samples were collected after a single dose during the PK lead-in period and following multiple oral doses during the study. Plasma PK samples were assayed for quantification of sitravatinib. The lower limit of quantification was 0.05 ng/ mL. PK parameters were determined using a noncompartmental analysis approach including maximum (peak) concentration (C max ), time to reach C max following drug administration (t max ), area under the plasma concentration-time curves (AUCs), apparent total clearance of the drug from plasma after oral administration, apparent volume of distribution during the terminal phase after administration and terminal elimination half-life (t 1/2 ). PD effects were examined by analyzing VEGF-A ligand and soluble (s)-VEGF-R2 levels in patients' plasma samples collected before and after sitravatinib administration.
Clinical activity was assessed by objective response rate (ORR) according to Response Evaluation Criteria In Solid Tumors (RECIST) v1.1. Additional endpoints included duration of response (DoR), progression-free survival (PFS) and overall survival (OS) in phase 1b cohorts of patients based on diagnosis and tumor molecular alterations. An exploratory post-hoc analysis of patients with non-squamous NSCLC who experienced disease progression on prior checkpoint inhibitor (CPI) therapy was performed.
Disease status was evaluated according to RECIST v1.1 at baseline and every three cycles in phase 1, and every two cycles (6-week intervals) in phase 1b. Assessments were performed until objective disease progression was documented or until subsequent anti-cancer therapy started (see Supplementary Information [Sect. 1.3] for details).

Statistical analyses
The mTPI method [7] was applied for dose escalation. Assumptions applied in establishing the mTPI method included the involvement of up to 30 patients in each regimen explored, a 0.3 probability of DLT at the MTD and an acceptable variance around the MTD of ± 0.05. At least three patients were planned for each cohort, safety permitting.
A DLT was defined as a grade ≥ 4 hematologic abnormality lasting ≥ 4 days; grade 3 thrombocytopenia with clinically significant bleeding; febrile neutropenia; clinically significant grade ≥ 3 non-hematologic AEs not related to underlying malignancy; intolerable grade 2 AEs; or toxicity resulting in an inability to deliver 80% of the dose during the first treatment cycle.
The safety population included all patients who received ≥ 1 dose of sitravatinib. The DLT evaluable population included all phase 1 patients who had taken ≥ 80% of the assigned doses of treatment and were evaluated for toxicity 21 days in the first cycle or had experienced a DLT in cycle 1. The PK evaluable population included all patients with sufficient concentration-time data for PK parameter evaluation. The modified intent-to-treat (mITT) population included all phase 1b patients who received ≥ 1 dose of study drug.
Cohorts of patients defined by tumor molecular markers were evaluated using an optimal Simon 2-stage design. Additionally, an exploratory analysis to describe the ORR in patients with NSCLC was performed.
DoR, PFS and OS were reported descriptively and summarized using the Kaplan-Meier method. DoR was defined as the time from first documentation of objective tumor response (complete response [CR] or partial response [PR]) until first documentation of disease progression per RECIST 1.1 or death (any cause). PFS was defined as the time from first dose of study treatment until progressive disease as defined by RECIST 1.1 or death (any cause). OS was defined as the time from first dose of study treatment until death (any cause).

PK and PD analyses
The PK evaluable population comprised 53 patients from the phase 1 and phase 1b cohorts; 40 patients participated in both the PK lead-in and cycle 1 portions, while seven patients participated only in the PK lead-in period and six patients participated only in the cycle 1 PK portion. A few patients in phase 1b receiving 120 mg sitravatinib also participated in the PK lead-in. After single oral administration of 10-200 mg under fasting conditions, sitravatinib was steadily absorbed with a median t max ranging from 3.02-8.87 h and arithmetic mean t 1/2 ranging from 42.1-51.5 h. After multiple oral administrations of 10-150 mg sitravatinib QD under fasting conditions, median t max,ss ranged between 2.00-8.13 h. At the proposed clinical dose (120 mg QD), the interpatient variability for C max and AUC τ,ss was ~60%. Steady-state appeared to have been reached by cycle 1 day 8, and C max,ss and AUC τ,ss accumulation ratios ranged from 1.82-6.89 and 2.13-8.34, respectively. Peak to trough ratios (PTR) in plasma for sitravatinib concentrations at steady state ranged from approximately 1.5-2.1-fold. Sitravatinib exposure (C max and AUCs) appeared to increase in an approximately dose-proportional manner following single-and multiple-dose administration from 10-200 mg, based on a statistical power model where the 95% confidence interval [CI] of the slope estimate for these PK parameters included the value of 1. Figure 2 shows the change in plasma concentration of sitravatinib over time after single and multiple doses. Key PK parameters are in Supplementary Table S3.
PD analysis demonstrated a concentration-dependent modulation of each PD marker with a percent change from baseline for VEGF-A determined as a 200% increase (Supplementary Fig. S1). Based on the EC50 (30.9 ng/mL) from an exposure-response analysis, 120 mg sitravatinib QD is expected to achieve an approximately near maximal effect on the drug target VEGF-R2.

Clinical activity
In the overall phase 1b mITT population, the ORR was 11.8% (19/161), with all responses being PRs (Table 3).   Table S4). For patients with NSCLC, the following molecular alterations were reported: RET alterations in 24 patients, MET alterations in 12 patients, CBL alterations in ten patients, Chr4q12 amplification in four patients and AXL, KDR and NTRK alterations in one patient each. The ORR for patients with NSCLC with a molecular alteration of  interest was 13.2% (7/53), while that for patients with nonsquamous NSCLC and prior CPI experience (exploratory analysis) was 4.2% (1/24) ( Table 3).

Discussion
Sitravatinib is a potent inhibitor of several RTKs that act as oncogenic drivers, including RET, TAM receptors and split kinase family receptors. This first-in-human phase 1/1b study demonstrated that sitravatinib had a manageable safety profile with AEs consistent with on-target inhibition and clinical activity was observed in selected populations.
Evaluation of sitravatinib in the phase 1 dose escalation stage resulted in a recommended phase 1b dose of 150 mg daily based on first cycle observations. However, after sequential evaluations of both 150 and 120 mg sitravatinib in phase 1b, 120 mg emerged as the recommended dose for further exploration based on a lower number of discontinuations, serious TRAEs and grade ≥ 3 TRAEs, compared with 150 mg.
Here, the PK profile of sitravatinib was characterized in patients with advanced solid tumor malignancies following single and multiple daily oral administrations from 10-200 mg. Under fasting conditions, sitravatinib was steadily absorbed with a median t max ranging from 3.02-8.87 h and arithmetic mean t 1/2 ranging from 42.1-51.5 h. At 120 mg QD, the between-patient variability for C max and AUC τ,ss was ~60%. Steady-state appeared to have been reached by cycle 1 day 8 and exposure (C max and AUCs) appeared to increase in a dose-proportional manner. PTR in plasma for sitravatinib concentrations at steady state ranged from approximately 1.5-to 2.1-fold, demonstrating a relatively small difference in steady-state C max and C min . The long t 1/2 and low PTR strongly support a once-daily dosing regimen for sitravatinib. Regarding PD effects, the magnitude of increase in VEGF-A and decrease in s-VEGF-R2 following sitravatinib treatment is consistent with effectively targeting the VEGF-R family and with the effects observed for other agents targeting VEGF-R, including sunitinib, axitinib and cabozantinib [8][9][10].
Modest clinical activity of sitravatinib was demonstrated in the overall phase 1b population (ORR 11.8%), where almost 60% of patients had received ≥ 3 prior systemic therapies. The ORR for patients with NSCLC with a molecular alteration of interest was 13.2%, which is lower than that reported for next-generation therapies selectively targeting a single kinase, such as MET or RET [11,12]. A post-hoc exploratory analysis of patients with NSCLC who experienced disease progression on prior CPI therapy showed that these patients did not gain a clinically meaningful benefit from sitravatinib monotherapy alone (ORR of 4.2%). Overall, these results suggested that sitravatinib, as a monotherapy, did not have significant anti-tumor activity in the analyzed cohorts, including NSCLC. However, sitravatinib is being investigated in combination with CPIs, based on its immunomodulatory role of the tumor microenvironment (TME).
CPI therapy is now established as a breakthrough treatment for various solid tumors, including NSCLC. Although many patients benefit from this treatment, some patients experience disease progression and develop resistance to CPIs through various mechanisms, such as the establishment of an immunosuppressive TME. Previous studies have revealed that targeting TAM receptors has an immunomodulatory effect on the TME, particularly involving polarization of tumor-associated macrophage populations [13]. Additionally, it has been demonstrated that targeting VEGF or VEGF-R decreases the number of immunosuppressive cells, such as regulatory T cells and myeloid-derived suppressor cells (MDSCs), in tumor models and patients with cancer [14]. Therefore, the role of sitravatinib in the modulation of the TME has been further explored. Preclinical data demonstrated that sitravatinib could modulate the TME by affecting macrophage polarization through inhibition of the expression of IL-4-stimulated arginase 1 (a marker of M2 polarization) [5]. Additionally, sitravatinib inhibited expression of the M2 markers arginase 1, YM-1 and Fizz-1 upon stimulation with conditioned media from murine cancer cells -a source of TAM receptor ligands -and reduced immunosuppressive cell populations, such as MDSCs and M2 macrophages, in vivo [5]. Notably, these changes facilitated a T effector cell response and augmented the effects of anti-programmed death (PD)-1/PD-ligand-1 (anti-PD-1) therapy in these xenograft models [5], and it was therefore hypothesized that the combination of sitravatinib with an anti-PD-1 agent, such as nivolumab, may have a synergistic clinical effect. This hypothesis was tested in a phase 1 window-of-opportunity trial evaluating sitravatinib monotherapy followed by sitravatinib combined with nivolumab in oral cavity cancer [15]. Sitravatinib monotherapy resulted in a less immunosuppressive TME with a reduction in MDSCs and repolarization of macrophages from the M2 to the M1 phenotype [15,16]. Additionally, sitravatinib followed by the combination with nivolumab for one cycle prior to surgery resulted in tumor reduction for all patients, including one CR [15].
Based on these preliminary data, the anti-tumor efficacy of sitravatinib with CPI therapy has been explored in the MRTX-500 phase 2 study, which evaluated sitravatinib plus nivolumab in advanced NSCLC and indicated encouraging results in patients who had progressed on, or after, prior CPI therapy [17]. These promising data have led to the evaluation of sitravatinib plus nivolumab compared with docetaxel in patients with non-squamous NSCLC in the ongoing phase 3 SAPPHIRE study (NCT03906071) [18]. Additionally, another phase 3 study (NCT04921358) [19] is evaluating sitravatinib plus tislelizumab (a PD-1 inhibitor) compared with docetaxel in patients with locally advanced or metastatic NSCLC.

Conclusion
In this study, the PK profile of sitravatinib was well characterized, indicating a steady absorption following oral administration and an appropriate t 1/2 for a once-daily dosing regimen. Sitravatinib had a manageable safety profile and demonstrated modest clinical activity in patients with heavily pretreated advanced solid tumors. Ongoing studies are evaluating sitravatinib in combination with other agents, such as anti-PD-1 inhibitors, in multiple tumor types, including NSCLC.